In a welded steel pipe, the quality of a welded portion is very important, and on-line flaw testing of the welded portion is ordinarily carried out by angle beam testing in a manufacturing process. In the technique, an ultrasonic wave is obliquely incident on an inspection surface of the sample to detect the flaw on the inner/outer surfaces of the sample, and the flaw inside the sample based on the reflected wave from the flaw. Ordinarily, a reflection technique method using an ultrasonic beam of 5 MHz having a refraction angle of 45° is applied to, for example, a electric resistance welded pipe, and flaws of the order of millimeters, for example, incomplete penetrations, burn through, and cracks due to inclusion, and the like are detected.
In contrast, recently, since very severe quality is required to the welded steel pipe, it is required to detect flaws smaller than conventional ones. For example, it is required to detect cold joint flaws and minute penetrators in a electric resistance welded pipe and to detect blow holes and the like in a laser welded pipe, and these flaws have a very small size of several tens to several hundreds of micron meters. Further, as a position of occurrence of flaws, they may occur in any location from an inside surface to an outside surface along a welding line. The incident point of the ultrasonic beam may be different from the return point depending on the flaw position. Since flaws are not detected often by ultrasonic flaw detection technique used practically up to now due to influence of them, a technique capable of detecting flaw more accurately is required.
The following conventional techniques are disclosed as techniques of detecting the minute flaw in the welded steel pipe.
Japanese Unexamined Patent Application Publication No. 60-205356 improves a penetrator detection capability in an angle beam testing by using a point focus type probe having a frequency of 8 MHz or higher.
Further, Japanese Unexamined Patent Application Publication No. 11-183446 improves a detection capability by forming a focus beam by an array probe so that blow holes can be detected by scanning from the inside surface to the outside surface of a welded portion by a sector scan.
Further, Japanese Unexamined Patent Application Publication No. 61-111461 detects cold joint flaws, which are mixed as a group of minute FeO of several micron meters or less, by causing an ultrasonic wave to be incident on a welded portion from the outside surface of a pipe at incident angle from 0° or more to 20° or less while setting the frequency of the ultrasonic wave from 25 MHz or more to 500 MHz or less.
Further, Japanese Unexamined Patent Application Publication No. 7-35729 detects blow holes of 0.1 mm or more using a plurality of point focus type probes, which have a frequency of from 20 MHz to 80 MHz and are disposed such that a focus position has a pitch of 3 mm or less from just above a seam.
It is noted that the description will refer to Japanese Unexamined Patent Application Publication No. 4-274756 and the document titled “Ultrasonic flaw detection series (II) Ultrasonic flaw detection method for welded steel pipe”, edited by Iron and Steel Institute of Japan, pp. 28-31, 1988.
However, the problems described below still remain even in the techniques disclosed above.
First, Japanese Unexamined Patent Application Publication No. 60-205356 has a problem in that many channels are necessary to detect the flaws in entire area in the depth direction of a welded portion (wall thickness direction of the steel pipe) without omitting them because the beam width of the focused ultrasonic wave is narrow and thus an equipment cost become expensive and further in that when a pipe size is changed, a position adjustment and the like are troublesome. Further, when a flaw is not a blow hole shape and is a plane shape as that in a penetrator and a cold joint as well as a flaw is located in a wall thickness inside portion, it is difficult to detect the flaw because the reflection wave travels in a direction different from the incident direction.
Further, in Japanese Unexamined Patent Application Publication No. 11-183446, since only one array probe is necessary as well as the setting can be electronically carried out when a size is changed, it can overcome the former problem shown in Japanese Unexamined Patent Application publication No. 60-205356. However, the latter problem is still remains unsolved.
Further, when a flaw shape is a plane shape as described above, since an upset is applied to a seam portion, in, for example, a electric resistance welded pipe, a flaw has a very narrow width of 100 μm or less when viewed from just above the seam. Accordingly, the reflection wave from the flaw is actually very weak even in the techniques of Patent Documents 3 and 4, and thus it is often difficult to detect the flaw. Further, since an area of about 1 to 2 mm in the vicinity of a surface echo is made to a dead zone owing to reverberation of the surface echo, a problem arises in that when a flaw is located in the vicinity of an outside surface, it cannot be detected.
As described above, a technique for detecting the minute flaws of about several hundreds of micron meters or less, which occur in a welded portion of a welded steel pipe in a pipe-axial direction, nondestructively, accurately, stably, and online, is not established except a C-scan technique for detecting it off-line by a sample cut out from the welded portion.